(CNN) — H5N1 bird flu has been identified in a pig in the United States for the first time, the US Department of Agriculture said Wednesday.
The USDA and Oregon veterinary officials are investigating bird flu cases in a backyard farming operation that had a mix of poultry and livestock, including pigs, the agency said.
“The livestock and poultry on this farm shared water sources, housing, and equipment; in other states, this combination has enabled transmission between species,” it said in a news release.
After H5N1 was identified in other animals on the farm, five swine were euthanised for testing; two tested negative, and results are still pending for two others.
The farm has been quarantined, and other animals are under surveillance.
However, it was not a commercial farm, and “there is no concern about the safety of the nation’s pork supply as a result of this finding,” USDA’s Animal and Plant Health Inspection Service said.
H5N1 is a type of influenza that’s rare in humans but is highly contagious and deadly in several species of animals, including poultry and dairy cattle, raising fears that it could mutate and become a virus that preys on people, too.
Scientists have been concerned that H5N1 might spread to pigs, which are considered “mixing bowl” species for flu viruses because they carry the same kind of receptors on cells in their lungs as humans and birds.
Some previous flu infections in pigs have allowed influenza viruses to change rapidly and develop new capabilities.
The 2009 H1N1 flu pandemic is believed to have been sparked by a virus that mutated in pigs in Mexico before it jumped to people.
Across the United States, more than two dozen people have tested positive for H5N1 flu this year, and nearly all of them have reported exposure to infected dairy cows or chickens, according to the US Centers for Disease Control and Prevention.
Some infectious disease experts said they wanted more clarity about the nature of the swine case in Oregon.
Dr. Michael Osterholm said he was wondering whether the virus was picked up on a swab of the pig’s nose or if there was evidence of deeper infection in the lungs.
“I think they use the word ‘infection’ too prematurely, because it very well may be just an environmental contamination of the nose. And so we need to get that, those data from that,” said Osterholm, an infectious disease expert who directs the Center for Infectious Disease Research and Policy at the University of Minnesota.
He noted that a study published in 2023, in which researchers tried to infect pigs with H5N1 by swabbing their noses with the virus and feeding them contaminated food, was largely unsuccessful.
Only one of eight pigs that were experimentally dosed with the virus later showed evidence of the infection in their blood, demonstrating that the pigs had “high resistance” to the infection, the study authors wrote.
Osterholm said he was glad that scientists were conducting additional studies on the animals in the latest case to learn more.
Veterinarians who track infections that spread between animals and people said it’s not too surprising that pigs would be infected on the same farm as infected birds.
“Sequencing of the virus will be important but it’s likely the circulating bird strain,” said Dr. Scott Weese, a veterinarian and chief of infection control at the University of Guelph in Ontario, in an email to CNN.
“I’d be more concerned if this was dairy cow-associated because mammal-to-mammal spread would be more concerning.
“I assume this is a dead-end spillover, but it highlights the potential issues” for cross-contamination of different species on farms, Weese said.
“Those issues would be greater with larger commercial farms, with more chance for pig-to-pig transmission” and greater chances of other flu viruses being present and swapping genes, he added.
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